The present invention relates to an ultrasonic probe, and in particular relates to an ultrasonic probe which has a shielding case.
An ultrasonic probe is used as an ultrasonic wave receiving source, for example of a medical ultrasonic diagnostic apparatus. As this type of ultrasonic probe, there is one where the periphery of a piezoelectric plate is covered with a shielding case.
FIG. 4 is a longitudinal (widthwise) cross-sectional view of a conventional ultrasonic probe.
A conventional ultrasonic probe U comprises, for example, a piezoelectric plate (piezoelectric element) 10, an inner case 20, a shielding case 30, an outer case 40, and an acoustic matching layer 50. The piezoelectric plate 10 has driving (excitation) electrodes on the opposite main faces. Regarding the driving electrode on the front face side, a lead wire 60 which is a connecting portion thereof, is extended to the back face side. Here, a pair of the lead wires 60 are connected to the back face side of the piezoelectric plate 10.
Moreover, the inner case 20 has a step portion 20a on its aperture side, which holds the periphery of the piezoelectric plate 10. A backing material 70 which has a damping function, is formed on the back face side of the piezoelectric plate 10, for example by pouring a synthetic resin into the inner case 20. The shielding case 30 is provided on the periphery of the inner case 20, and has the aperture end face thereof made close to the wave transmission reception face so as to cover the periphery of the piezoelectric plate 10. Accordingly, this prevents the entry of external noise to the inside of the ultrasonic probe U, and acts to prevent erroneous diagnosis.
Furthermore, the outer case 40 is provided on the periphery of the shielding case 30. The acoustic matching layer 50 is provided on the aperture end face of the piezoelectric plate 10, the inner case 20, the shielding case 30, and the outer case 40, so as to cover the whole ultrasonic wave transmission reception face. The thickness of the acoustic matching layer 50 which is provided facing the front face of the piezoelectric plate 10, is generally set to ¼ of the wavelength λ of the ultrasonic wave. Moreover, in order to ensure the safety of a living body in contact with the wave transmission reception face, as shown in FIG. 4, the acoustic matching layer 50 is provided over the whole wave transmission reception face.
(See Japanese Unexamined Patent Publication (KOKAI) No. 2000-115891, and to Japanese Unexamined Patent Publication (KOKAI) No. 2004-57806).
However, in this type of conventional ultrasonic probe U there is a problem in that, due to the shielding case 30, it is not possible to maintain good voltage endurance. That is, in the ultrasonic probe U having such a construction, since the aperture end face 30a of the shielding case 30 has a small width and a high voltage is applied, an electric field is concentrated in the vicinity (at the tip) of the aperture end face 30a. Accordingly, there is a problem of destruction of the acoustic matching layer 50, particularly that which is located in the vicinity of the aperture end face 30a, shown by the imaginary line P in FIG. 4.
In this kind of ultrasonic probe, generally, even if a voltage of 4 KV is applied, without causing destruction of the acoustic matching layer 50. Voltage endurance under normal operation should be ensured. For example, inspection is performed by applying a voltage of 4 kV to the front face of the acoustic matching layer 50, with the driving electrodes on both the main faces of the piezoelectric plate 10, and the shielding case 30 externally connected and made earth potential.
An object of the present invention is to provide an ultrasonic probe which has a shielding function, and in which excellent voltage endurance is maintained.